1. Field of the Invention
The present invention relates to springs having contacts which are used as contacting parts of variable resistors, trimmers, motors, cameras, potentiometers, etc., and more particularly to a method of producing a spring having multicontacts (hereinafter referred to as "a multicontact spring").
2. Description of the Prior Art
For a contacting spring to have a high reliability, it is generally necessary that it have a plurality of contacts. Miniaturization of electrical apparatus using a contacting spring requires a corresponding decrease in the size of the contacting spring. Accordingly, for miniaturization of electrical apparatus it is necessary to increase the density of the number of contacts per unit width of a spring and to minimize the contact spacing (i.e. distance between adjacent edges of two adjacent contacts). For example, there is a demand to produce a multicontact spring which is 1 mm in width and has five contacts which are less than 0.2 mm in width, respectively. However, such a multicontact spring has not been successfully produced by conventional methods, i.e., a photo etching method and a blanking method.
In accordance with the conventional photo etching method, a multicontact spring is produced by the steps of:
coating both surfaces of a spring blank with a photo resist;
shining light on the photo resist through a negative pattern film;
removing parts of the resist;
etching the parts of the spring blank which are not coated with the resist, and;
removing the remaining resist. An illustration of the thus produced multicontact spring can be seen in FIGS. 1A and 1B. The contact spacing of the multicontact spring produced by the above mentioned photo etching method is usually limited by the thickness of the spring, in view of the so called side etching (i.e. etching of a part under the resist). Furthermore, it is difficult to make the contact spacing less than 0.1 mm. Finally, since each of the corner edges 1 of a contact is formed in an acute angle, as illustrated in FIG. 1B, the corner edges 1 may damage a contact surface of a member which comes in touch with the multicontact spring. In a case where a multicontact spring is produced by the photo etching method disclosed in the Japanese Patent Publication No. 52-46889, the spring material is a precious metal alloy (i.e. platinum and silver alloy), so that the etching rate is small. In this regard, it is possible, after photo etching a copper-base alloy blank, to plate a precious metal alloy for the contact material on the copper-base alloy spring, but the production process of such a multi-contact spring is rather complicated. Accordingly, such a photo etching method of producing a multicontact spring is not suitable for mass production.
An illustration of a multicontact spring produced by the conventional blanking method (i.e. a press working method) can be seen in FIGS. 2A and 2B. In the case of the conventional blanking method, the contact spacing of the produced multicontact spring is limited to approximately 0.2 mm by the conventional die producing technique. Furthermore the contact spacing increases as the thickness of the spring material increases. Finally, as the width of a contact decreases, an undesirable deformation (e.g. twisting or curling) may occur in the contact.
It is also known that the above mentioned multicontact spring having a plurality of contacts per unit width of the spring can be produced by a welding method. An illustration of the multicontact spring, which is a so called multi-wire spring brush, produced by this method can be seen in FIGS. 3A, 3B and 3C. In accordance with the welding method, the multicontact spring is produced by the steps of arranging a number of wires 2 contacting parts between a base plate 3 and an additional plate 4, welding the wires 2 and the plates 3 and 4 by spot welding or soldering, and then, cutting the wires 2. As the diameter of each of the wires used as contacts decreases, the density of the number of wires per one spring can be increased, but more uniform diameters and higher quality wires are required. Furthermore, as the number of the wires increases, it is possible to easily generate a small difference in the shapes of the welded wires, so that the quality of the multicontact springs produced by the welding method is not stable.